1. Field of the Invention
The present invention relates to a turbo decoding apparatus and a decoding iteration count controlling method in turbo decoding.
2. Description of the Related Art
A turbo coder outputs transmit data itself, data obtained by converting the transmit data to a convolutional-coded data (first redundant bit) and data obtained by interleaving and convolutional-coding the transmit data (second redundant bit).
On the other hand, the turbo decoder performs soft decision decoding (estimation of transmitted data) using two decoders corresponding to two convolutional codes. Then, the turbo decoder feeds back the decoding result of one decoder to another decoder and iterates this processing. Then, the turbo decoder finally makes a hard decision to confirm “1” and “0” of the data.
The reliability of turbo decoding increases as the number of iterations of soft decision decoding increases.
However, as the number of iterations increases, the time required for decoding also increases and a processing delay becomes not negligible.
Furthermore, when the number of iterations of soft decision decoding increases, the load on processing such as path calculation based on a Viterbi algorithm.
This raises a problem of increasing power consumption of the circuit as well. Therefore, it is an important problem when iterations of decoding should be terminated.
The following methods are proposed as methods for controlling the number of decoding iterations:                {circle around (1)} Method whereby a soft decision value of immediately preceding processing is compared with a soft decision value of current processing and iteration is terminated when there is no difference (European Patent Laid-Open Publication EP0755122A2).        {circle around (2)} Method whereby an output value (soft decision value) of a soft decision decoder is compared with a limit value and iteration is terminated when the output value is close to the limit value or when saturation of data changes is detected in a large number of bits (Unexamined Japanese Patent Publication No. 2000-216689).        {circle around (3)} Method whereby a test bit is added to data to be coded during coding, the presence or absence of any error is checked using the test bit every time soft decision decoding is performed and it is decided based on the test result whether further decoding should be performed or not (Unexamined Japanese Patent Publication No. 2000-183758).        
The above-described technologies {circle around (1)} and {circle around (2)} decide whether not to terminate iterations focused on a soft decision value (information of a plurality of bits), and therefore require complicated calculations.
Furthermore, the technology {circle around (3)} conducts a check on the test bit for each decoding, and therefore the processing is complicated.
It is an object of the present invention to simplify processing of deciding whether or not to terminate iterations of soft decision decoding in turbo decoding and alleviate the processing load on the turbo decoding circuit.